I. Field of the Invention
The present invention relates to adaptors for mechanically coupling plastic pipes to other members such as metal pipes, unions, and mechanical joints, and more particularly, to such adaptors for mechanically coupling polyolefin water pipes to cast iron mechanical joints.
II. Description of Prior Art
Typical water pipe systems use very rigid pipes, such as ductile or cast iron, concrete and/or PVC pipes which mate with fittings such as mechanical joints. While such pipes are advantageously strong, many are very heavy and cumbersome to work with, and in the case of iron suffer from corrosion. PVC pipe may crack when flexed and has numerous mechanical joints which present potential for leak sites. A substitute for such pipe is flexible polyolefin-type plastic pipes which are lightweight, easy to work with, corrosion resistant and are fused together at the joint to form a continuous pipe to thereby minimize leakage. Polyolefin pipe, however, suffers from certain drawbacks that have impeded widespread use in water pipe systems and the like.
In particular, mechanically mating the end of a polyolefin pipe to some of the other members in a water system, and particularly to metal mechanical joints, presents significant difficulties due, for example, to the cold flow properties of the polyolefin-type plastic. More specifically, the ends of such a pipe tend to deform under pressure leading to an inadequate seal at the mechanical connection. Moreover, the pipe end may work loose from the mechanical joint due, for example, to the greater expansion/contraction rate of that type of plastic as compared to other more rigid pipes. In such a case, the connection pulls apart.
To reduce the drawbacks of using polyolefin-type of plastic pipe, therefore, there has been a need to stiffen the pipe end so that it will not deform under pressure nor work lose from the joint. One approach utilizes a length of such pipe one end of which is adapted to couple to the polyolefin pipe of the water system in conventional manner such as butt fusion. The other end of the adaptor includes an integral flange to be gripped by the mechanical joint components. To form the flange, the end of the pipe may be extruded with a pipe wall that is much thicker than a typical pipe wall, which thickened end is then machined to define a nominal thickness pipe wall with the integral flange thereon. Such an approach is costly and of questionable vitality in that the flange may be insufficient to properly retain the pipe in sealing engagement with the mechanical joint.
Another approach force-fits a rigid tube such as a steel tube inside the polyolefin pipe at the pipe end in sealing engagement with the inner diameter of the plastic pipe wall. The rigid tube act as a stiffener at the pipe end. And while the stiffening tube is believed to provide better resistance to pipe end deformation, the polyolefin pipe end may still come loose from the mechanical joint.